Operating conditions of a catalyst system in an exemplary industrial process catalytic reactor cover a relatively small range of variables. The flow ranges in an exemplary industrial process are no more than 4 or 5:1, the inlet flow and mixing conditions are well defined, and the reaction zones are also closely controlled to produce temperature profiles that are maintained within a narrow range. Start conditions are carefully controlled to assure the catalyst and reactor are performing according to prescribed conditions. This type of operation leads to catalyst system designs that have less demanding requirements compared to operations of similar processes where temperature and performance controls must be held tightly over a wide range of dynamic operating conditions.
One of the most familiar deviations from the exemplary industrial process reactor is the automotive exhaust emission control catalyst system. In this system, the start and operating conditions are significantly different. Temperature difference and reactant ratio from start to operating conditions can vary rapidly. This catalyst system also operates with wide ranges of throughput, which can vary as much as 50:1, with space velocities in excess of 100,000 hr−1, and high heat release (significantly higher than the exhaust emission converter) over the operating range. Because of their application, catalytic reactors must be compact as well. This demands the development of creatively designed catalyst systems that can be prepared by methods commercially conceivable.
One commercially available approach for exhaust emission control catalysts employs honeycomb monoliths for the reactor. In the operation of honeycomb monolith catalytic reactors on vehicles, thermal profile and conversion are maintained over the wide engine operating conditions, without raising system back pressure, by combining the catalyst bed structure variables, associated with the honeycomb monolith, with catalyst preparation procedures. By increasing the honeycomb cell or channel density, i.e., decreasing the cell size and reducing the wall thickness, the fluid dynamics of the reactant gases and reactions taking place over the entire operating range are improved over larger cell sizes, thus helping to maintain reasonably well-controlled temperature profiles and subsequently providing better durability.
Through the use of honeycomb monolith catalyst systems, catalyst type and loading over the length of the catalyst/converter flow path can offer reaction and temperature profile control, while simultaneously meeting the conversion required via catalyst loading and control of material availability during catalyst preparation. Improved catalyst materials, preparation procedures, and structure were developed concurrently with refinement of the catalyst structure in achieving this higher performance.
Drawbacks exist, however, for honeycomb monolith catalyst systems in fuel cell fuel processor system applications. The heat flux (heat release rate and quantity) and the relative ratio of size to degree of reaction complexity associated with an automobile exhaust catalytic reactor are relatively small compared to the primary reactor of a fuel cell fuel processor. Also, automobile honeycomb monolith exhaust catalytic reactors often detrimentally develop laminar flow through large sections of the straight, continuous, channels of their honeycomb structure. This creates mass transport problems, and the only effective means for distributing heat is axially, along the length of the channels. If any over-temperature condition develops in the inlet section, where turbulence does exist, or if any blockage or unequal distribution of inlet feed exists, there is no means by which the reactants can migrate from one channel to another in a single monolith, or redistribute the reactant flow some distance downstream from a blockage.
Modification of the honeycomb monolith assembly provides some improvement. By providing many “slices” of a honeycomb monolith, wherein the channel alignment of each slice is offset, conditions closer to continuous mixing and redistribution phenomena take place. This approach also has drawbacks, however, in that washcoating and catalyst loading are performed after assembly, which can result in non-uniform washcoating and catalyst loading throughout the offset monolith. After-assembly washcoat and catalyst loading is both extremely difficult and cost ineffective to both prepare and subsequently to retain the catalyst structure without physical damage to the washcoat or catalyst layer(s).
Another catalyst structure that has been considered for automotive and fuel processor applications is the foam support. This structure potentially provides the desired tortuous flow path that can assist in maintaining turbulent mixing of the reactants through the catalyst bed, thereby enhancing mass and heat transport properties. The disadvantage of this support structure is similar to the “sliced honeycomb monolith” in that washcoating and catalyzing the surfaces requires forcing a slurry of the washcoat and/or catalyst material through its webbed structure after assembly. Washcoating these supports uniformly throughout the structure can be very problematic, particularly when cell density is relatively high. In addition, these supports have been found to be very non-uniform in both cell size and cell distribution. This results in areas where blockages occur because of fabrication faults, making coating and distribution activity less controlled. A further disadvantage is that backpressure through these monoliths is higher than for the honeycomb monoliths.
Based on the above, there is a need for tailored catalyst systems that meet the desired activity of a fuel cell fuel processor (reactor) over a wide range of operating conditions. Catalyst system requirements for an onboard hydrocarbon (i.e., gasoline, LPG or NG) conversion processor have both similarities and significant differences from the previously mentioned examples. Similarities include: 1) the choice of catalyst and catalyst loading must be commensurate with the reaction processes and cost requirements; and 2) heat transfer and control of temperature are critical to maintaining life and conversion selectivity.
An optimal catalyst system design for fuel processor operation should provide a variety of flow paths throughout the length of the catalytic reactor, in order to induce turbulent flow to accommodate the reaction flux over the entire set of operating conditions, without compromising (1) the catalyst loading or type, (2) life of either the catalyst or reactor, (3) pressure drop, or (4) performance, i.e., conversion/selectivity. Uniformly controllable washcoat and catalyst loading is not available with the honeycomb or foam support design catalytic beds. A catalyst manufacturing method is therefore required which also incorporates controlled, but passively variable flow paths throughout the catalyst bed to promote turbulent flow throughout, the ability to control washcoat and catalyst loading prior to assembly of the bed, and an assembly which permits either the washcoat or the catalyst or both washcoat and catalyst to be varied through the catalytic bed.